This invention relates to silicon-aluminum alloys and also includes a microelectronic packaging material comprising an alloy as aforesaid.
Information Processing is typically achieved by semi-conductors (e.g. silicon or gallium arsenide) that are interconnected, powered, cooled and protected by "packaging". The materials used in the packages, and onto which the electronic devices are mounted and enclosed, include a wide range of low cost plastics, as well as common metals and alloys (e.g. Al, Cu, Ti). However, for demanding applications (high power/high frequency), more sophisticated alloys, composites or ceramics are increasingly being specified. For example, as the density of circuitry gradually increases, the amount of heat generated also increases and, therefore one critical function of the packaging material is to dissipate the heat from the electronic devices, otherwise their efficiency deteriorates and their "life" is reduced. Furthermore, thermal mis-match between the electronic components and the heat sink to which they are attached must be minimized in order to prevent stressing and subsequent failure of the brittle electronic devices and/or solder joint connections. Consequently, package materials which exhibit low and controlled coefficients of thermal expansion (CTE), combined with high thermal conductivity, are essential. An additional requirement for many defence, aerospace and space applications is the minimisation of weight.
Pure aluminium and conventional aluminium alloys have been popular as electronic packaging materials for the dissipation of heat due to their high thermal conductivities in the range 200-235 W/mK. However, with the higher levels of integration used in multi-chip modules and microwave integrated circuits there are demands that cannot be satisfied by aluminium packages. Namely, the thermal expansion mismatch caused by the high CTE of these materials which are in the range 22-24 ppm/.degree. C., (compared to gallium arsenide and silicon which have CTE volumes of approximately 6 ppm/.degree. C. and 3 ppm/.degree. C. respectively).
Other materials such as an 54% Fe-29%Ni-17%Co alloy, also known as KOVAR (a Registered Trade Mark of The Carpenter Technology Corporation), with a CTE of 5.8 ppm/.degree. C. reduce the thermal expansion mismatch with semi-conductor materials. However, KOVAR has a low thermal conductivity (15-17 W/mK), a high density (8.2 g/cc) and an inadequate specific stiffness (17 GPa.cm.sup.3 /.sub.g). Thus, KOVAR is not the optimum material for efficient thermal management.
Refractory metals such as molybdenum and tungsten have both high thermal conductivity and low CTE values. These metals are normally mixed with copper to form W--Cu and Mo--Cu composites which can be machined. However both these composites have a high density, and therefore there is a weight penalty.
Weight considerations are particularly important in avionics and space applications. One method of producing lightweight electronic packaging materials combined with high thermal conductivities and low CTEs is via aluminium metal matrix composites (JOM, July 1992, p.24-28). Particulate silicon carbide is a common ceramic reinforcement due to its low coefficient of thermal expansion, high thermal conductivity, and low cost, compared to other ceramic particulates such as aluminium nitride. However, silicon carbide loadings in the range 65-75 vol % are required to obtain aluminium metal matrix composites with the low CTE required for electronic packaging.
Methods for the manufacture of Al/SiC metal matrix composites (MMC) can be classified as follows:
(i) liquid state processes in which the ceramic particulate is added to the molten matrix alloy, agitated to prevent SiC settling due to density differences, and cast to shape. However, this method is limited to SiC levels up to 30 vol %, because of problems with fluidity, and to certain aluminium alloy matrices which do not react with SiC to form aluminium carbide which is detrimental to mechanical properties and corrosion resistance. PA1 (ii) solid state processes in which the matrix alloy is blended with ceramic particulate and pressed. In a practical sense these processes are limited to SiC reinforcements of approximately 40 vol % placing a limit on the CTE reduction available. PA1 (iii) infiltration processes in which a green SiC compact is infiltrated with the molten matrix alloy. Very high SiC particle loadings can be achieved by infiltration processes (up to 75 vol %). Since the SiC--Al is non-wetting, infiltration is assisted using pressure or by conditioning the SiC particle surfaces to assist infiltration by capillary action. Whilst successful in achieving the SiC loading required for CTE matching with semi-conductor materials, the metal matrix composites are extremely difficult to machine. In addition, Al/SiC MMC is also difficult to metallize, to join, and can be prone to gas leakage at the high vacuums required for certain applications. PA1 (iv) another route is spray forming, in which an aluminium-based alloy is melted and inert gas atomized and ceramic particulate is injected into the metal spray which co-deposits to form a metal matrix composite. However, it is not possible to introduce the required high loading of SiC (65-75 vol %) by this method. Such attempts have resulted in excessive porosity, particle clumping, and poor SiC loading reproducibility. PA1 (a) inhomogeneous distribution of the injected phase in the matrix, PA1 (b) excessive porosity, PA1 (c) high oxide content (because of the use of fine powder which has a large surface area), and PA1 (d) low yields (because the fine particulate is carried away with the atomizing gas). PA1 50-90 wt % Si PA1 10-50 wt % Al PA1 0-10 wt % other alloying additions wherein the silicon and aluminium form substantially continuous phases, the silicon phase is made up of randomly oriented, fine crystals of silicon in the microstructure of the alloy material; the coefficient of thermal expansion is in the range of 4.5-11 ppm/K; and the thermal conductivity is greater than 100 W/mK. The other alloy additions are deliberately added and exclude trace elements. The additions may include: PA1 Magnesium--up to 2 wt %--for refining the silicon phase. PA1 Copper--up to 5 wt %--forming a tertiary phase with a low coefficient of thermal expansion. PA1 Iron--up to 8 wt %--forming a tertiary phase with a low coefficient of thermal expansion. PA1 Zirconium--up to 0.5 wt %--for strengthening the aluminium matrix. PA1 Atomizing gas--Nitrogen PA1 Gas to Metal ratio--4 m.sup.3 /kg PA1 Spray Distance--700 mm PA1 The billet size is suitably 10 kg of 150 mm diameter.
Another method for the production of aluminium alloys with a low coefficient of expansion is spray forming of hypereutectic aluminium-silicon alloys. Instead of introducing an exogeneous reinforcing ceramic phase or phases in the form of particulate or fibres, the reinforcing phase, silicon, is formed `in-situ` by nucleation and growth from the liquid during solidification of the spray formed deposit. The advantage of this method is that the endogeneous reinforcing phase, silicon, is in perfect atomic contact with the aluminium matrix because the solid-state phases which constitute the alloy are derived from the same source, namely, the alloy in the liquid state. We have spray formed Al--Si alloys with silicon contents up to 50 wt %. For example, we published a paper in the Second International Conference on Spray Forming in 1993 relating to such alloys where the main objective was to produce Al--Si alloys up to 50 wt %. for thixoforging, (i.e. forging in the semi-solid state to form structural wear-resistant products). Also our U.S. Pat. No. 5,143,139 discloses the spray forming of Al-20Si alloys. However, it was found that it was not possible to hot work materials containing more than 35% silicon.
In EP-A-411577, there is disclosed a method of producing aluminium based alloys containing silicon where aluminium silicon alloys with up to 15 wt % Si are melted and fine silicon powder, preferably not more that 10 .mu.m mean particle size, is injected as particulate to produce spray formed deposits with total silicon contents up to 55 wt %. According to the first aspect of that invention, "silicon which is soluble in aluminium is deliberately sprayed in the form of solid particles and mixed into the aluminium alloy. Thus it is possible to produce an aluminium based alloy having a high content of silicon without increasing the melting temperature of the aluminium alloy." An example given was the preparation, melting and spraying of an aluminium alloy containing 15 wt % silicon and the simultaneous deposition of silicon particles of 3 .mu.m mean particle size in amounts that would result in total silicon contents of 35 wt %, 45 wt % and 55 wt %. The pouring temperature of the Al-15 wt % Si alloy was quoted at 650.degree. C.
We have found that the injection of fine particulate at such high volume fractions leads to:
Accordingly, we believe such a method is impracticable.
Another prior proposal is contained in WO94/11138 which discloses a silicon-based alloy where an alloy is rapidly solidified into powder by atomisation or by melt spinning and milling prior to hot isostatically pressing. Because of the large difference in melting points between silicon and aluminium, the final formed product will have discrete silicon "islands" within an aluminium matrix.
It is also known that silicon-aluminium alloys can be produced by melting and casting, see for example, the German article "Gefuge und thermische Volumenanderungen von Al--Si-Liegierunger" of Chanyuang Gan and Erhard Hornbogen. However, in the compositional range of interest (51-90 wt %) the as-cast microstructure is characterized by mainly large, discrete, faceted, high aspect ratio primary silicon crystals which impair mechanical properties and machinability. For example, in FIGS. 1 and 2 of this application there is shown the microstructure of chill cast A170% Si. Primary silicon crystals appear black against the light grey Al--Si eutectic constituent. The primary silicon particle size, which appear as single acicular crystals, is of the order of millimetres which results in a very anisotropic microstructure. This makes it quite unsuitable for the application to electronic packaging. For example, the walls and bases of the enclosures which go to make up electronic packages are typically 1-3 mm thick and, therefore, with a chill cast material it would be possible to have one single silicon crystal passing through the base or side walls of the package. This would make the material extremely difficult to machine to the fine surface finishes required for metallization, because the silicon crystal would be susceptible to fracture in a single direction along a preferred crystallographic plane. In addition, because of the large silicon crystal size, the local CTE and thermal conductivity of the material may vary widely depending upon whether aluminium or silicon is in contact with the chip or chip carrier. It is generally considered that these alloys have no engineering applications and are only used as master-alloys for liquid metal processing industries or steel de-oxidation.